Vacuum gap discharge device having grooved electrodes for thermal insulation



VACUUM GAP DISCHARGE DEVICE HAVING GROOVED ELEGTRODES FOR THERMALINSULATION Filed July ll, 1966 Jan. 30, 1968 J. M. I Ar-'FERTY 3,366,825

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United States Patent O ABSTRACT F vTHE DISCLOSURE Vacuum arcdevicesincluding a pair of primary arc electrodes adapted to sustain anarc therebetween. Improved current carrying capacity results from arcdispersal means at the periphery of one arc electrode, including anannular slot at the electrode periphery defining a peripheral ridgewhich isuniformly heated to vapor emission temperature to avoidformation of anode spots.

The present invention is a continuation-impart of my copendingapplicationyser. No. 422,373, filed Dec. 30, 1964 and assigned to thepresent assignee, now abandoned.

This invention relates to improved vacuum gap devices and particularlythose of the triggered vacuum gapand triggered vacuum switch type and toimproved electrode configuration therefor.

Vacuum switches and vacuum gaps, particularly triggered vacuum gaps haverecently undergone great im provements. A number of such 'devicesgenerally referred to as vacuum gap devices are presently of importantcommercial considerations. vIn most of these vacuum gap devices alimiting factor as to the current carrying capacity lies in the abilityof the anode to withstand destructive melting caused by the formation ofintense anode spots. Similar destructive melting also occurs lat thecathode eletcrode, although the vanode destruction is the mostextensive. f Accordingly, it is an object of the present invention toprovide improved vacuum gap devices having electrode configurationswhich facilitate the carrying `of eX- tremely high currents withoutdestructive melting ofthe anodes thereof.

A further object of the invention is to 'provide triggerf able vacuumswitch and fixed vacuum gap devices in whichl the electrodes are adaptedto carry high values of current between arc initiation and arcinterruption' without destructive melting thereof. v

A further object ofthe present invention is to provide improved vacuumgap devices which are' suitable for utilization over a greater number'ofcircuit interruptions or arc estabilshments than devices of the priorart.

In accord'with one feature of the presentinvention I provide a vacuum'gap device including a pair of primary electrodes defining therebetweena vacurn gap. These electrodes are located within an evacuable envelopewhich envelope is composed partially of metal and at least in part of `ahigh voltage insulating lsubstanfce so as to provide electricalvisolation between the primary electrodes to prevent sh'ort circulitingof the gap defined tlhereby. InV one embodiment of my invention the mangap electrodes are provided with an annular groove closely adjacent tothe external periphery thereof, which groove provides a portion thereofwhich is, as far as thermal conduction characteristics are concerned,partially isolated from the remainder of the main electrode in orderthat the isolated portions thereof may achieve a highertemperature caus-3,366,825- K Patented Jan. 30, 1968 ICC 2 y ing a substantially `uniformemission'ofvapor. therefrom and thepreventionl of -destructive`spots.7Inaccord with a further feature ofthe present invention the electrodesofthe wacuum'devices thereof lare so shaped'afs to puoi/ide a centralportion at which the gap'between'theprima'ry electrodes is larger thanthe gap lat the peripheryJ thereof so as to facilitateA the location ofthe arc discharge' between the peripheral portions of the 'electrodes atwhich the thermally isolated portions are located.Inaccord with afurther feat-ure of the present invention a trigger electrode is locatedwithin the central portion of at least one of'the electrodes andis-connected so as to cause the `injection of a highly ionized gaseousplasmaintoy the gap between the main electrodes tofcause thel breakdownthereof-upon the initiation of a voltage pulse thereto. rThe novelfeatures believe-d characteristic'of thepr'esent invetnion are setforthin the appened claims. IThe invention itself, together with furtherobjects andv advantages thereof may'lbe more readily understoodliby areference to the appended drawing in which:

FIGURE 1 is a vertical cross-sectional view of a fixed gap triggerablevacuum gap device constructed in accord with the present'invention, andi FIGURE 2 is a vertical cross-sectional view of the trig`` gerelectrode utilized inthe device 0f FIGURE 1'.- 'i The vacuum gap deviceof FIGURE f1 includesv an envelope represented generally as 1`containing ttherein a pair'of primary arc electrodes 2 andS'deni'ngtherebetween a primarygap 4. A trigger'electrode 6'havin'g a trigger gap7 is concentrically located withina central aperture 8 of electrode 2and extends to substantially to the center of the primary gap 4.Envelope 1 is comprised of a substantiallycylindrical metallic ymember 9having a central bulbous portion 10 which. is symmetrical `along thelongitudinal axis thereof,

a llirst flanged metallic end piece 11 having a central aperturethereinand a second yflanged metallic apertured end'plate 12 also havinga central aperture therein. A pair of substantially cylindrical,centrallyk apertured `insulator bushings 13 and v14 each of which has' alarge outside diameter` portiony and a smaller outsider diameter portionconnected by an annular shoulder serve "as 'insulating members for thedevice and `are assembled with 'the smaller outside diameter portionthereof extending in'- wardly with the annular shoulder `resting againstthe planar exterior surface of aperturedlend`plates`11y and 12respectively. The lcentral apertures'within insulator bushings 13 and`14 are closed by the insertion therewithin of annular cylindricalelectrode support members `1'5and 16r respectively; The interior ofcylindric-alelectrode sup port member 15 is'hollow and'is sealed tovaccurn by the insertion therein and sealing thereto 'of ltriggerelectrode assembly v6. Each'of the central cylindrical aper-l tures inbushings 13 and 14 are slightly larger than the outside diameterofelectrode support memb ers15 and 16 forvadded insulation propertiesandtoprovide an insulator surface -not 'likely to be short circuited byconf densed metal vapor. A vacuum seal is madefbetween these membersby4the brazing thereto of `annular seal members 17 and :18 respectivelyeach of which lhasqa larger diameter portion which fits. tightlyover-the outside diameter ofthe respective insulator bushing' and asmaller diameter portionjlwhich lits tightlyy over the exteriorporltionofthe respective electrode `support member.

The-rnaterials comprising the devices illustrated f in FIGURE 1 aresubstantiallyfasfollows. Main envelope member 9 may be a metal whichwillwithstand high temperatures and which possessesv sufficientyphysical strength as to constitute lthe outside 'body of thede'vice, asfor example stainless steel. `Flanged end plate 'members 11 `and 12 andflange seal members 17 and 184a1e` comprised of a material which makesgood vaccum-tight seals to ceramic bushings 13 and 14 and to metallicmembers 9, 15 and 16 respectively and may conveniently be a Fernico orKovar alloy generally utilized in electric discharge devices for thispurpose. Bushings 13 and 14 are comprised of a high temperature g-asimpermeable ceramic material, as for example high density alumina (94%or higher or A1203). Electrode support members 15 and 16 are preferablycomposed of a highly conductive substantially gas-free material, as forexample OFHC copper Premium Grade. No special precautions need be takento render this material gas-free, other than vacuum firing prior toassembly. Primary electrodes 2 and 3 are composed of highly purifiedcopper or other high vapor pressure material, 4as for example any of thematrials .set forth in Lee and Cobine Patent No. 2,975,256 issued Mar.14, 1961. This material is rendered substantially free of `all gases andgaseous-forming compounds by vacuum melting, as for example by repeatedzone reining steps .so as to reduce the concentration of gas andgas-forming impurities therein to a figure of less than one part in 106.

Electrodes 2 and 3 are preferably provided with at least one arcdispersion means 19 although only the anode need be so provided. Arcdispersion means 19, in the illustrated embodiment comprises an annularslot which is cut from the arcing surface 8 of electrode 2, for example,so as to provide a vertical surface 20 and a thermally substantiallyisolated annulus 21 on electrode 2. The dimensions of arc dispersionslots 19 is related to the material from which the electrodes are formedand to the gap distance. In general the slot should be located farenough in from the periphery so that the peripheral isolated region soformed is raised to a vaporemittin'g temperature without destructiveboiling of vapor at too rapid a rate. The depth of the slot is notcritical but is related to the thickness of the electrode body.Naturally, the slot depth should be sufficient to substantially increasethe surface-to-volume ratio'of the peripheral region and to s`oconstrict the path of thermal conduction as to allow for the controlledheating of the peripheral region as described hereinbefore. For a givengeometry, as the chosen electrode material, the width of the slot 19varies indirectly with vapor pressure and directly with thermalconductivity of the material. As one example of dimensions, for a 21/2diameter, 1/2" thick electrode of copper, with a gap width of Mzoperated on l60 c.p.s. alternating current voltage, slot 19 was Ms" wideand 1A, deep and the width of the peripheral region 21 Was M3". Arcdispersion means may take other forms. Thus, for example, a series ofslots may be drilled in a circular pattern along a radius only slightlyless than the radius of the electrodes. Preferably, such slots should beinterconnected to provide for arc rotation and for a uniform dispersionof the arc. Alternatively, a plurality of annular slots could be c'ut atdifferent radial distances from the center of the arc to create severalperipheral vapor emitting ridges.

The trigger electrode assembly 6 of FIGURE 1 of the drawing isillustrated in greater detail in vertical cross section in FIGURE 2 ofthe'drawing. In FIGURE 2 the trigger is composed of a hollow cylindricalmetallized ceramic base member 22 which has a narrow circumferentialgroove 23 cut therein, which may be tapered as shown, near the inwardextremity thereof. Means for sealing the trigger electrode in avacuum-tight seal to the interior of electrode support member 15 areprovided in the form of an annular flange member 24 having an extendedshoulder and an apertured collar member 25 which rests thereupon and issealed thereto. Both of these members are conveniently constructed of ametal suitable for .making metal to ceramic seals, as for example one ofthe general class of metal alloys known as Fernicos. A first hollowcylindrical shield member 26 having a first counter-bore 27 therein anda second counterbore 28 of greater diameter also therein rests uponcollar member 25. The smallest diameter of cylindrical member 26 makeselectrical contact with metallized ceramic cylindrical member 22. Alirst metallic sleeve member 29, which is beveled at one end thereof tofit the bevel in the annular groove 23 in ceramic cylinder member 22,slides over member 22 and is aligned with the inward or lower bevel ofthe annular groove therein. A second cylindrical sleeve 30, which islikewise beveled at one of its ends to match the groove 23 incylindrical member 22, slides over ceramic -member 22 and is alignedwith the outward or upward bevel of groove 23 therein. A metallic capand shield piece 31 having a re-entrant portion 32 .and a centralaperture 33 rests upon the upper end of ceramic cylindrical member 22and Within the interior end of second sleeve member 30. A trigger leadwire 34 centers the central aperture within ceramic trigger supportmember 22 and is affixed to the re-entrant end cap 31 at the uppermostportion thereof. This lead may conveniently be of nickel or any othermaterial conventionally utilized for providing lead wires in electricdischarge devices. Members 24 and 25 are conveniently selected as aFernico or equivalent. The remaining metallic members 26, 29, 30 and 31are conveniently selected as a refractory metal, as `for example,tungsten or molybdenum. Appropriate vacuum-tight seals are made to themating surfaces members of all of the ceramic and metal parts byconventional, well-known techniques. When assembled, the triggerelectrode assembly is lowered into the open end of electrode supportmember 15 and rests upon a counterbore shoulder therein where angemember 24 is sealed in a vacuum-tight seal thereto. Prior to nalassembly the portions of metallic sleeve members 29 and 30 which are tobe exposed between the upper portion of cylindrical shield member 26 andcylindrical end cap and shield member 31 are coated with a material, asfor example a hydride of titanium, zirconium, hafnium, yttrium, erbiumor other rare earth metals which hydrides serve as a source of ionizablegas which m-ay be emitted upon the initiation of a voltage pulse betweensleeve members 29 and 30.

The device of the present invention may be fabricated in accord withstandard ceramic and metal tube technology in which case the device isassembled and the various members thereof are sealed into assemblies andsub-assemblies and the device is finally sealed leaving only plug 35unsealed. The device is then placed in an outgassing furnace and raisedto a temperature of, for example, 800 C., and held at this temperaturefor one or two hours in order to cause outgassing and the removal ofsorbed gases from all of the constituent parts thereof. After suflicientoutgassing, and while the device is still at an elevated temperature anatmosphere of, for example, hydrogen is introduced through plug 35 andthe device is sealed. Since, in the case of a titanium hydride -coatingupon met-al sleeves 29 and 30, a large fraction of the hydrogen has beenevolved therefrom, hydrogen is taken up from the furnace atmosphere torecharge the trigger electrode and to cause the establishment of avacuum within the sealed envelope. In order to enhance this operation areservoir for an active gas, as for example hydrogen, may be provided inthe form of reservoir 36 which -comprises two larger metallic,preferably .stainless steel annular washers 37 and 38 with an interposedsmaller outside diameter w-asher 39 of the same material. The assemblyof these three washers creates an annular cavity 40 which is filled withgranular active gas absorbing material, as for example titanium 41, theopening thereto being covered with a wire mesh 42 which may convenientlybe 200 mesh stainless steel screen. Since, in the case of titaniumhydride used as a gas reservoir, the titanium hydride of the reservoiris substantially exhausted of hydrogen by the outgassing process, afterthe device has been sealed by plug 3S and the device cools to roomtemperature substantially all of the remaining hydrogen gas within thedevice is taken up by the titanium to form titanium hydride. Since themaximum concentration of hydrogen in the titanium hydride at roomtemperature is substantially represented by the formula TiH1 73 asuicient quantity of titanium is provided within reservoir 41 to ensurethat not all of the hydrogen which could be absorbed thereby is presentwithin the envelope prior to sealing. Thus, a hard vacuum of at least -5millimeters of mercury is provided by the absorption of hydrogen by thetrigger electrode and by the reservoir.

An alternative method for fabricating devices in accord with the presentinvention is substantially disclosed and claimed in my copendingapplication Ser. No. 417,562 tiled Dec. 11, 1964, now U.S. Patent No.3,331,961 assigned to the assignee of the present invention. In accordwith the method disclosed and claimed in my copending application, thematerials are assembled unsealed and the charged material utilized forthe trigger electrode and the reservoir are initially supplied withinthe device. The device is then placed in an atmosphere of the active gascontained within the reservoir, as for example, hydrogen in the case oftitanium hydride, and the temperature is elevated to a temperature tosufficiently outgas the component parts of the device and tosimultaneously cause pure hydrogen to be evolved from the triggerelectrode and from the reservoir to further purge the device of allimpurities other than pure active gas. The temperature of the device isthen further raised to cause sealing of the ceramic and metal partstogether after which the sealed device is allowed to cool to roomtemperature to provide the requisite vacuum of less than 10*5millimeters of mercury.

The operation of a triggered vacuum gap in accord with the presentinvention and as illustrated on FIGURE 1 of the drawing is quite similarto that of the triggered gaps of the prior art as is disclosed withgreat particularity in my Patent No. 3,087,092, issued Apr. 23, 1963.Briefly stated, this action is substantially as follows. Electrodes and16 are connected to a source of high voltage, as for example, a primarypower transmission line to be protected from overvoltages. A highelectric eld is therefore established within gap 4 between electrodes 2and 3. When, because of a high voltage transient which could damage theequipment to be protected, a preselected pulse of voltage is suppliedbetween trigger lead 34 and electrode 2. Since electrode 2 is connectedto the lower sleeve 29 abutting the triggered gap and the trigger leadis connected to the upper sleeve 30 abutting the trigger gap, atransient high voltage is applied across the trigger gap. Due to the lowbreakdown potential required at the interface between metal and ceramic,a spark discharge is init-iated between sleeves 29 and 30. The heat ofthis discharge immediately causes the evolution of an active gas, as forexample hydrogen from an active gas storing substance, as for exampletitanium hydride, which comprises the coating upon these two sleeves. Asthe hydrogen is evolved the hydrogen molecules are ionized, thus causingthe arc discharge across the triggered gap to be intensified, causingthe release and ionization of a large quantity of hydrogen gas. A pulseof a hydrogen ion-electron plasma is thus injected into the gap betweenthe main electrodes 2 and 3 and the main arc is broken down within amatter of microseconds or less.

Since the initial breakdown between primary electrodes 2 and 3 occurs inthe immediate vicinity of the trigger electrode where the path lengthbetween the two electrodes is long and since the vacuum arc isessentially vapor starved, the natural tendency of the arc is toconserve vapor causing a propultion of the arc out into the smallergap-length region between the two electrodes. This operation so far isconventional.

In my experimental work I have determined that the prime cause of thefailure of vacuum gap devices, as for example the triggered vacuum gapillustrated in FIGURE l of the drawing is the destructive melting of theanode electrode by virtue of the formation of anode spots. As usedherein an anode spot is intended to indicate the footpoint of the arcbetween the primary electrodes where it is anchored to or terminates atthe anode electrode.

Under normal operation of a vacuum arc discharge the anode electrode issurrounded by a space charge of negative charge carriers, namelyelectrons. Electrons which are attracted to the positive or anodeelectrode must fall through the potential represented by the spacecharge surrounding the anode. At the anode, all of the discharge currentis carried by electrons impinging thereupon. This electron bombardmentof the anode is primarily responsible for anode heating. Anode heatingis produced both by the kinetic energy of the incident electronscorresponding to the anode voltage drop and by the heat of condensationof the electrons. As the arc current between the primary electrodes isincreased the density of the space charge sheath surrounding the anodeincreases, thus increasing the anode potential drop. This increases theenergy of electrons impinging upon the anode, causing its temperature tofurther increase.

As the current density is increased further, a point is eventuallyreached at which a given local area of the anode, randomly selectedbecause of anode geometry and other considerations or by instabilitiesin the arc is suciently heated as to cause the release of metal vapor.This metal vapor is immediately ionized by the high velocity electronsstriking the anode. This is because, in all but the very shortest ofvacuum arcs the anode potential drop exceeds by several times theionization potential of the metal vapor. The ions created by suchionization tend to neutralize the space charge in the localized area ofthe anode emitting metal vapor. This neutralization of the space chargecauses the anode voltage drop at the localized area to decrease causingmore electron current to tiow t0 the anode in this area. A cyclicreaction thus takes place with a continued build-up of localized anodecurrent, 4re- Y sulting in a hot spot at the localized portion and aconstriction of the arc at the anode. This is known as an anode spot.The intense local heating produced by the anode spot causes destructivelocalizing melting of the anode. If the vacuum discharge device isoperating on unidirectional current, the damage can be great upon theformation of the spot. Even if the device is operating on alternatingcurrent and only one-half cycle of time elapses between striking andre-striking of the arc, the anode spot formed within this time can causedestructive erosion of the anode electrode.

The utilization of the electrode structure of the present inventiongreatly diminishes or eliminates such anode spot formation. Thus, inaccord'with the present invention the anode is caused to emit vapor overan extended area in a controlled fashion. This is achieved by providinga large area of the anode electrode (or of both electrodes if there issome uncertainty as to the electrode which will be the anode uponstriking of an are) which so emits to prevent localized `spot formation.In the illustrated embodiment of the invention shown in FIGURE 1 of thedrawing, the entire peripheral region 21 of the electrode is caused toso emit. As is described hereinbefore, conservation of vapor principlestend to propagate the arc to the exterior portions of the electrodes.When the arc located between the peripheral regions of the mainA gapelectrodes, peripheral region 21 is raised in temperature to a higherdegree than the remainder of the electrode. This is due to two effects.Firstly, heating is increased because the surface-to-volume ratio ofperipheral portion 21 is larger than the remaining portion of theelectrodes since the impinging electrons may strike the grooved verticalsurface, the top surface, and the vertical outside surface of theperipheral region of the electrode. Additionally, since the peripheralregion of the anode electrode is connected to the main body of theelectrode only by a restricted crosssectional area under the peripheralslot, the conduction of heat away from the peripheral portion of theelectrode is less than would be in the absence of slot 19. Accordingly,peripheral region 21 becomes uniformly heated to a temperature to causemetallic vapors to be emitted at the edges of the outer ring unifor-mlyaround the electrode, thus preventing the formation of a single anodespot with destructive melting and destruction of the anode electrode.Further accentuation of this effect may be achieved by the location of aradial magnetic field normal to the longitudinal axis of the device ofFIGURE 1 which tends to rotate the arc current conducting pathcircularly around the electrode, thus further reducing the probabilityof the formation of anode spots.

Although one embodiment, namely a peripheral groove, has beenillustrated and discussed in detail it will be appreciated that otherequivalent means to achieve the results contemplated by the presentinvention may be utilized. The necessary criterion to be met is that theouter periphery of the electrode be heated to a higher temperature thanthe remaining portion of the electrode. This is achieved by causingincreased surface-to-volume ratio for that portion and by causing arestricted thermal conduction path between that portion of the electrodeand the main portion of the electrode. As mentioned hereinbefore thismay be achieved by other expedients than the cutting of a peripheralslot as is illustrated in FIGURE 1.

' Although the vacuum gap device of the present invention has beenillustrated and described with reference to the specific embodiment of afixed gap triggerable device, the advantages of the present inventionmay be equally achieved by the incorporation of the disclosed electrodestructure in a fixed gap device which is not triggerable or in a vacuumswitch of the circuit interruptor or recloser type in which oneelectrode is fixed and the other is movable from a circuit closingposition to a circuit breaking position and in such devices whether thedischarge is triggerable or whether the interrupter is of theconventional vacuum type.

While the invention has been described in detail herein in accord withcertain preferred embodiments thereof, many modifications and changestherein may be effected by those skilled in the art. Accordingly, it isthe intention in the appended claims to cover all such modifications andchanges as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A vacuum gap device comprising: lan envelope evacuated to a vacuum ofat least 5 mm. of mercury and having at least a portion thereof composedof a high voltage insulator; a pair of primary electrodes constitutingan anode electrode and a cathode electrode disposed within said envelopeand adapted to define therebetween a breakdown gap, at least said anodeelectrode having arc dispersion means at the periphery thereofsufficient to inhibit the formation of destructive anode spots, saidperipheral region of said anode electrode being a portion ofthe arcingsurface thereof, being substantially uniformly in poor thermal contactwith the remainder of said electrode and having a high substantiallyuniform surfaceto-volnme ratio, such that said peripheral region underarcing conditions becomes uniformly heated to a temperature at which arcsustaining vapor is uniformly e-mitted; and means for connecting saidelectrodes in circuit with an electrical load.

2. The vacuum gap device of claim 1 wherein both electrodes are fixed todefine a fixed dimension gap.

3. The vacuum gap device of claim 1 wherein at least one electrode ismovable between circuit making position in contact with said otherelectrode and a circuit breaking position out of contact with said otherelectrode and defining a gap therewith.

4. A vacuum gap device comprising: an envelope evacuated to a vacuum ofat least 105 millimeters of mercury and having at least a portionthereof composed of a high-voltage insulator; a plurality of primaryelectrodes constituting an anode electrode and a cathode electrodedisposed within said envelope and adapted to define therewith abreakdown gap, at least said anode electrode having arc-dispersion meansat the periphery thereof sufficient to prohibit the formation ofdestructive anode spots, said peripheral region of said electrode beinga portion of the arcing surface thereof, being in poor thermal contactwith the remainder of said electrode and having a high surfaceto-volumeratio, such that said peripheral region under arcing conditions becomesuniformly heated to a temperature at which arc-sustaining vapor isuniformly emitted therefrom; trigger means associated with one of saidelectrodes and comprising a trigger electrode having therein a secondarygap and having located immediately adjacent said gap a quantity of amaterial having chemically bound therein a quantity of an activeionizable gas which is thermally evolved therefrom upon the initiationof a spark breakdown across said trigger gap; means for connecting aprimary voltage between said primary electrodes and means for deliveringa pulsed voltage to said trigger electro-de independent of the primaryvoltage to cause breakdown of the primary gap.

5. The vacuum device of claim 4 wherein both primary electrodes arefixed to define a fixed gap.

6. The vacuum device of claim 4 wherein one primary electrode is fixedand the other primary electrode is movable between a closed,contact-making position and an open contact breaking position.

7. The vacuum device of claim 4 wherein both primary electrodes aresubstantially circular discs with substantially flat arcing surfaces andthe arc-dispersion means comprises a peripheral annular slot in thearcing surface of at least said anode electrode.

8. A vacuum gap device comprising: an envelope evacuated to a vacuum ofat least 10-5 millimeters of mercury and having at least a portionthereof composed of a high voltage insulator', a pair of primaryelectrodes constituting an anode electrode and a cathode electrodedisposed within said envelope and adapted to define therewith abreakdown gap, at least said anode electrode having an annular slot inthe arcing surface thereof near the periphery thereof so as to define aperipheral surface arcing region of very small radial dimension ascompared with the radial dimension of the entire electrode and having ahigh surface-to-volume ratio, said peripheral region under arcingconditions becoming uniformly heated to a temperature at whicharc-sustaining vapor is uniformly emitted therefrom; means associatedwith one of said electrodes to inject a gaseous plasma into said primarygap upon receipt of a pulsed signal and comprising a trigger electrodehaving thereon a trigger gap, and a quantity of a hydride of an activemetal immediately adjacent to such trigger gap so that hydrogen gas isthermally released therefrom upon the initiation of a spark breakdownacross said trigger gap; and means for connecting said primaryelectrodes in circuit with an electrical load; and means for supplying asource of pulsed voltage to said trigger electrode to cause thebreakdown of the primary gap.

9. The vacuum device of claim 8 wherein both primary electrodes arefixed to define a fixed gap.

10. The vacuum device of claim 8 wherein one primary electrode is fixedand the other primary electrode is movable between a closed,contact-making position and an l open contact breaking position.

11. The vacuum arc device of claim 8 wherein the hydride utilized tostore hydrogen is titanium hydride.

12. The vacuum arc device of claim 8 wherein the hydride utilized tostore hydrogen is yttrium hydride.

13. A vacuum gap device comprising: an envelope evacuated to a vacuum ofat least 105 mm. of mercury and having at least a portion thereofcomposed of a high voltage insulator; a pair of primary electrodesconstituting an anode electrode and a cathode electrode havingsubstantially planar arcing surfaces disposed in closed juxta- 9 l0position Within said envelope to define therebetween a References Citedbreakdown gap, at least said anode electrode having an UNITED STATESPATENTS annular slot in the arcing surface thereof at the peripherythereof, said slot isolating a peripheral surface arcing 31163173412/1964 Lee' region of very small radial dimension as compared with 5the radial dimension of the entire electrode and having FOREIGN PATENTSa high substantially uniform surface to volume surface 1,306,918 9/ 1962France.

ratio, said peripheral region under arcing conditions becoming uniformlyheated to a temperature at which arc- JAMES W, LAWRENCE, PrimaryExmminer. sustaining vapor is uniformly emitted therefrom to` preventthe formation of anode spots, and means for connectlo S- SCHLOSSER RJUDD, ASSSG'H Examinersing said electrodes in circuit with an electricalload.

